1. Field of the Invention
The present invention relates to an amplified photoelectric transducer having a novel pixel structure, an amplified solid-state image sensor including a plurality of such amplified photoelectric transducers arranged in a matrix, and a method for driving the amplified photoelectric transducer. More particularly, the present invention relates to an amplified photoelectric transducer capable of eliminating reset noise and obtaining a high-resolution video signal (image signal), an amplified solid-state image sensor including a plurality of such amplified photoelectric transducers, and a method for driving the amplified photoelectric transducer.
2. Description of the Related Art
Conventionally, an amplified solid-state image sensor has been proposed which has a plurality of pixels (or amplified photoelectric transducers) each having an amplification function and reading out a signal charge by a scanning circuit. For example, APS (Active Pixel Sensor) image sensors are known in the art. The APS image sensor employs an amplified photoelectric transducer-having a CMOS structure, and can be integrated with peripheral circuits such as a driving circuit or a signal processing circuit.
In the APS image sensor, however, it is necessary to provide a photoelectric transducer section, an amplification section, a pixel selection section and a reset section all within one amplified photoelectric transducer. Consequently, three or four n-type transistors are usually needed in addition to the photoelectric transducer section made of a photodiode (hereinafter, referred to also as a "PD").
FIG. 12 illustrates an amplified photoelectric transducer of an image sensor described in Mabuchi, et al., "1/4-inch VGA-compatible 330,000-pixel CMOS image sensor" in the Technical Report of the Society of Image Information Media, IPU 97-13, March 1997. The amplified photoelectric transducer employs a PD+3T pixel structure, where each pixel includes a PD and three transistors.
More specifically, an amplified photoelectric transducer 200 is formed within a region surrounded by a read-out clock line 11, a reset clock line 12, a signal line 13 and a power line 14. The amplified photoelectric transducer 200 includes a photodiode 5, an amplification section 1 formed of an n-type MOS transistor, a reset section 2 and a pixel selection section 3.
FIG. 13A illustrates a two-dimensional pattern of the amplified photoelectric transducer 200 illustrated in FIG. 12. FIG. 13B is a cross-sectional view taken along line B--B in FIG. 13A. Referring to FIG. 13A, the amplification section 1, the reset section 2 and the pixel selection section 3 are aligned in one vertical direction.
In FIG. 13A, the photodiode 5, the n-type transistors 1, 2 and 3, and the lines 11, 12, 13 and 14 correspond to those in FIG. 12. The photodiode 5 which functions as a photoelectric transducer is formed by an n-layer 21 in a p-type substrate 20.
As described above, the transistors 1, 2 and 3 illustrated in FIGS. 12 and 13A are all n-type MOS transistors, while the photodiode 5 is a p-n junction diode. Thus, the image sensor can be easily produced by a standard CMOS process.
In the image sensor having such a pixel arrangement, as illustrated in FIGS. 12, 13A and 13B, however, when the photoelectric transducer section (the photodiode 5) is reset, reset noise is inevitably generated from thermal noise of the reset MOS transistor 2.
More specifically, at the beginning of each frame period for storing a signal charge, the reference potential of the photodiode 5 randomly fluctuates. Where the value of the fluctuation is represented by ".DELTA.Q" based on the amount of charge, while the capacitance of one entire photoelectric transducer section including the gate capacitance of the amplification MOS transistor 1 is represented by "CP", then the fluctuation value .DELTA.Q can be represented as in Expression (1) below. EQU .DELTA.Q=.sqroot.(k.multidot.T.multidot.CP) (1)
where
k denotes Boltzmann's constant, and PA1 T denotes an absolute temperature
Therefore, when a signal charge is read out from an amplified photoelectric transducer for every frame period, the signal charge contains reset noise due to the fluctuation .DELTA.Q. The reset noise among various pixels within one frame or among various frames within one pixel has no correlation with each other, and the reset noise appears on the screen as spatially and temporally random noise.
In order to cancel out the reset noise, it is necessary to determine the difference between an output signal at the beginning of a signal charge operation and that at the end of the signal charge operation within one frame. This requires a relative difference, rather than an absolute signal amount, thereby requiring the initial potential of each pixel to be stored for the frame period. This in turn requires a frame memory, thereby presenting a disadvantage in that the system structure of the image sensor needs to be substantially large.
When a digital memory is used for the frame memory, a quantization error in an A/D conversion is inevitable. When an analog memory is used for the frame memory, other noise occurs when converting the signal charge into an analog value to be stored.
For these reasons, it has been nearly impossible to completely eliminate the reset noise, and the elimination of such reset noise has been an ultimate goal in the field of APS CMOS image sensors.